1. Field of the Invention
The present invention relates to photoelectric sensors, and more particularly, the present invention relates to improvements in photoelectric sensors that receive detection light and generate determination signals.
2. Description of Related Art
A photoelectric sensor generally includes a light projecting section that generates detection light, a light receiving section that receives the detection light, and a control section that controls a timing of the light projection and reception and generates a determination signal indicating the presence or absence of a detection target object based on a light reception amount. The photoelectric sensors are grouped into two types: an integral type in which a light projecting section and a light receiving section are arranged in one housing, and a separate type in which the light projecting section and the light receiving section are respectively arranged in separate housings. The photoelectric sensors are further divided into two types: a transmission type in which the detection light is blocked by the detection target object, and a reflective type in which the detection light reflected by the detection target object is received.
A determination signal is generated based on a comparison result obtained by comparing the light reception amount when detection light is projected with a predetermined determination threshold value. For example, in the transmission type photoelectric sensor, it is determined that a detection target object is present when the light reception amount is smaller than the determination threshold value. On the other hand, in the reflective type photoelectric sensor, it is determined that a detection target object is present when the light reception amount is larger than the determination threshold value.
Examples of conventional photoelectric sensors include a photoelectric sensor that is capable of adjusting the sensitivity by displaying a light reception amount and a determination threshold value and changing the determination threshold value (see, for example, JP H9-252242 A). Also, there is a photoelectric sensor that determines and displays a margin ratio indicative of how much margin a light reception amount has with respect to a determination threshold value (see, for example, JP 2006-292702 A). JP 2006-292702 A discloses changing a determination threshold value by adjusting a rotary operator.
However, the photoelectric sensors of the above types have such a disadvantage that the determination threshold value is not appropriately fixed and determination accuracy is reduced, when the photoelectric sensors are used in a situation where, in the presence or absence of a detection target object, an amplifier circuit is saturated which amplifies a light receiving signal indicating a light reception intensity.
For example, in a reflective type photoelectric sensor, a light reception amount is saturated in some cases when a distance to a detection target object is smaller than a specified value. Moreover, signal noise or ambient light is likely to affect the determination and the determination accuracy may be reduced when a difference in the light reception amount between the presence and absence of a detection target object is small.
On the other hand, there is known a photoelectric sensor in which adjusting light reception sensitivity of a light receiving section or a light emission amount of a light projecting section makes it possible to prevent an amplifier circuit for a light receiving signal from being saturated and to suppress reduction in determination accuracy. The light reception sensitivity is adjusted by changing an amplification factor of the amplifier circuit. For example, the amplification factor is adjusted by changing a resistance value of a trimming resistor. The trimming resistor is a variable resistor having a resistance value that changes according to a rotational position of an operator operated by a user.
Most of conventional photoelectric sensors, which adjust the light reception sensitivity using a trimming resistor, generate a determination signal using an analog circuit. The analog circuit compares a determination light receiving signal in which a light receiving signal is superimposed on a reference level with a level signal indicating a threshold level. Further, the photoelectric sensors of this type adjust, in many cases, the sensitivity by changing the reference level of the determination light receiving signal in order to ensure a wide dynamic range. Hence, displaying the signal level of the determination light receiving signal or the threshold level does not make it easy to grasp an adjustment state as to whether or not the sensitivity is appropriately adjusted.
In addition, a photoelectric sensor that changes a threshold level to adjust light reception sensitivity has a smaller dynamic range than a photoelectric sensor that changes a reference level to adjust light reception sensitivity. Thus, in this type of photoelectric sensor, the dynamic range is thus increased in such a manner that plural detection modes with different detection ranges are made switchable or the photoelectric sensor is equipped with a function of adjusting a light reception parameter other than a threshold level, such as an amplification factor of a light receiving circuit. However, this does not enable an intuitive adjustment because adjusting a threshold level and adjusting a light reception parameter for offsetting the dynamic range are separate processing.